The present invention relates generally to a torque transmission apparatus, and particularly to a motor vehicle torque transmission apparatus incorporating a multi-stage torque converter.
One application of torque converters is in the speed-changing power transmission of heavy duty vehicles. Generally, a torque converter comprises an impeller, a turbine and a stator. A means of providing multi-stage torque conversion through a converter having separate stators has been advanced recently. Such a multi-stage torque converter is capable of generating high-ratio stall torque.
FIG. 5 shows a conventional torque transmission apparatus including a multi-stage torque converter. The multi-stage torque converter comprises an impeller 1 connectable to an engine, a turbine 2, a first stator 3a and a second stator 3b. The turbine 2 is connected to a turbine shaft 4, and the first stator 3a is connected to a stationary shaft 6 through a one-way clutch 5. The turbine shaft 4 and the stationary shaft 6 are connected with each other through a transmission having gears 7, 8, 9, 10, and 11. The structure is such that the rotational direction of the turbine 2 is opposite to that of the first stator 3a.
FIG. 6 shows the r.p.m. versus output torque characteristics of the torque transmission apparatus having a conventional multi-stage torque converter. The response curve illustrates that high output torque is generated in the low r.p.m. range. Under these characteristics, when the r.p.m. reach a point N, the first stator 3a begins to rotate. In the range of r.p.m. above point N, the output torque characteristics are the same as for a conventional three-element torque converter which has a non-partitioned stator.
Heavy duty vehicles can entail higher-output torque requirements. The output torque in the low r.p.m. range can be increased wherein a high reduction ratio is established, as shown by a dashed line in FIG. 6; however, in consequence, the torque-drop occurring in mid-range r.p.m. (as indicated within the circled portion A in FIG. 6) is "steeper.". Moreover, to establish a higher reduction ratio in this way would mean using larger-diameter gears, increasing the overall apparatus size. This limits the extent to which the output torque can be increased in the conventional apparatus.
U.S. Pat. No. 4,924,978 discloses a power transmission having a reduction-gear apparatus coupled to a first stator of a multi-stage torque converter. Because the transmission has three speeds there is consequently less torque-drop in mid-range r.p.m. However, shocks occur at speed-changing stages in the transmission, wherein the output torque response is stepped. In heavy-duty vehicles in particular, such jolts can be severe.
It is possible to reduce the shock at the speed-changing stages by utilizing a stepless speed-change power transmission, as disclosed in U.S. Pat. No. 4,843,908. Engine power is transmitted therein to an output shaft through a belt and pulleys, which undergo high-torque loads. Belt type transmissions, however, are of limited strength, and are not suitable for use as the speed-change power transmission in heavy duty vehicles occasioning high-torque transmission demands.
A further consideration of relevance is that the torque converter/automatic transmission systems with which passenger cars are furnished are of very complicated structure, and jolting at transmission speed-changing therein has not proven easy to prevent.